Using¹⁰Be dating to determine when the Cordilleran Ice Sheet stopped flowing over the Canadian Rocky Mountains

Abstract

During the last glacial maximum the Cordilleran and Laurentide ice sheets coalesced east of the Rocky Mountains and geomorphological evidence indicates ice flowed over the main ridge of the Rocky Mountains between ~54–56°N. However, this ice flow has thus far remained unconstrained in time. Here we use in situ produced cosmogenic 10Be dating to determine when Cordilleran ice stopped flowing over the mountain range. We dated eight samples from two sites: one on the western side (Mount Morfee) and one on the eastern side (Mount Spieker) of the Rocky Mountains. At Mount Spieker, one sample is rejected as an outlier and the remaining three give an apparent weighted mean exposure age of 15.6 ± 0.6 ka. The four samples at Mount Morfee are well clustered in time and give an apparent weighted mean exposure age of 12.2 ± 0.4 ka. These ages indicate that Mount Spieker became ice free before the Bølling warming and that the western front of the Rocky Mountains (Mount Morfee) remained in contact with the Cordilleran Ice Sheet until the Younger Dryas.

The collapse of the Laurentide-Cordilleran ice saddle and early opening of the Mackenzie Valley, Northwest Territories, constrained by 10Be exposure dating

“…Initial separation of the CIS and LIS over the southern Mackenzie Mountains is not chronologically constrained, but further south (55°N), mountain summits over 2000 m asl became ice free as early as 15.6 ± 0.6 ka (Dulfer et al, 2021), consistent with our earliest ages on the deglaciation in the study area at Katherine Creek at 15.8 ka (17.1 -14 6 ka). The western margin of the LIS then stayed pressed against the eastern slopes of the Mackenzie Mountains during a period of ice sheet thinning between 14.9 and 14.3 ka (Fig.…”

“…In addition, recent studies in adjacent regions of the LIS also included a GIA correction (Norris et al, 2022;Reyes et al, 2022) and hence, to ensure comparability with these datasets, we apply a correction for GIA related changes to our dataset as well. The previously mentioned changes in atmospheric conditions following deglaciation work against the impact of GIA on calculated exposure ages, but the impact on exposure ages is likely an order of magnitude lower than that of GIA induced impacts (Staiger et al, 2007;Cuzzone et al, 2016;Ullman et al, 2016;Dulfer et al, 2021).…”

“…As most of the exposure ages post-date the LGM, the influence of katabatic winds and other atmospheric dynamics changes associated with the LIS and CIS ice sheets would have been brief and therefore limited (Staiger et al, 2007). Previous studies that have investigated the impact of changes in atmospheric mass distribution have found that it results in a younger exposure age calculation, but the impact is minimal (Cuzzone et al, 2016;Ullman et al, 2016;Dulfer et al, 2021). Coupled with the absence of any model at a suitable resolution, we choose not to make corrections for changes in the atmosphere.…”

The collapse of the Cordilleran–Laurentide ice saddle and early opening of the Mackenzie Valley, Northwest Territories, Canada, constrained by 10Be exposure dating

“…In addition, recent studies in adjacent regions of the LIS also included a GIA correction (Norris et al, 2022;Reyes et al, 2022), and, hence, to ensure comparability with these datasets, we apply a correction for GIA-related changes to our dataset as well. The previously mentioned changes in atmospheric conditions following deglaciation work against the impact of GIA on calculated exposure ages, but the impact on exposure ages is likely an order of magnitude lower than that of GIA-induced impacts (Staiger et al, 2007;Cuzzone et al, 2016;Ullman et al, 2016;Dulfer et al, 2021).…”

“…As most of the exposure ages postdate the LGM, the influence of katabatic winds and other atmospheric dynamics changes associated with the LIS and CIS ice sheets would have been brief and therefore limited (Staiger et al, 2007). Previous studies that have investigated the impact of changes in atmospheric mass distribution have found that it results in a younger exposure age calculation, but the impact is minimal (∼ 4 % of the GIA correction, Cuzzone et al, 2016; 1 %-5 % of the GIA correction, Ullman et al, 2016; ∼ 1 % of the GIA correction, Dulfer et al, 2021). Coupled with the absence of any model at a suitable resolution, we choose not to make corrections for changes in the atmosphere.…”